1. Field of the Invention
The present invention relates to a liquid ejection head that ejects liquid contained in a liquid chamber from a nozzle as liquid droplets, a liquid ejection apparatus, and a method for fabricating the liquid ejection head. In particular, the present invention relates to a technology that improves print quality while maintaining the rigidity of a nozzle sheet including the nozzle.
2. Description of the Related Art
A liquid ejection head that ejects liquid from a nozzle using an energy generating element has become widespread. For example, printer heads of inkjet printers are of this type, in which a pressure is applied to ink contained in an ink chamber using an energy generating element so that the ink is ejected from a nozzle as ink droplets. The ink droplets are deposited on a print paper sheet placed in front of the nozzle so as to form substantially circular dots in vertical and horizontal directions and represent an image or characters.
Under ideal conditions, the ink droplet is ejected from the nozzle of the printer head in a direction perpendicular to a nozzle sheet including the nozzle. However, in practice, the ejection direction of the ink droplet is usually not perpendicular to the nozzle sheet. If the ejection direction is not perpendicular to the nozzle sheet, the position of a deposited ink droplet on a print sheet is offset from the proper position. Thus, white streaking may occur on an image, and therefore, the quality of the image is degraded.
To prevent the occurrence of white streaking, the present inventors proposed a technology that changes the ejection direction of an ink droplet. In that technology, a plurality of heating elements (one type of energy generating elements) capable of being independently driven is arranged in an ink chamber. By independently driving the heating elements, the ejection direction of an ink droplet can be deflected (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-1364).
FIG. 14 is an exploded perspective view of a known printer head 111 described in Japanese Unexamined Patent Application Publication No. 2004-1364. In the drawing, an exploded nozzle sheet 117, which is bonded to a barrier layer 116, is shown. Also, for the sake of convenience of description, the printer head 111 is shown upside-down relative to the orientation typically used for the real printer head 111.
In the printer head 111, a substrate member 114 includes a semiconductor substrate 115 composed of, for example, silicon, and a heating element 113 formed by deposition on a surface of the semiconductor substrate 115. The heating element 113 includes left and right separated portions.
A barrier layer 116 is formed on the surface of the semiconductor substrate 115 on which the heating element 113 is formed. The barrier layer 116 serves as a member for forming an ink chamber 112. A nozzle sheet 117 serves as a liquid ejection member in which a plurality of nozzles 118 is formed. The nozzle sheet 117 is bonded to the barrier layer 116 so that the nozzles 118 face the heating element 113.
The ink chamber 112 is formed from the substrate member 114, the barrier layer 116, and the nozzle sheet 117 such that the substrate member 114, the barrier layer 116, and the nozzle sheet 117 surround the heating element 113. That is, as shown in FIG. 14, the substrate member 114 and the heating element 113 form a bottom wall of the ink chamber 112, the barrier layer 116 forms side walls of the ink chamber 112, and the nozzle sheet 117 forms a top wall of the ink chamber 112. Thus, the ink chamber 112 includes an opening in the lower right area in FIG. 14, through which ink is provided to the ink chamber 112 from an ink tank (not shown) connected to the printer head 111.
In the printer head 111 having such a structure, by heating the heating element 113, the ink in contact with the heating element 113 generates a bubble. The expansion of the bubble expels a certain volume of the ink. An ink having the same volume as the expelled volume is ejected from the nozzles 118 in the form of an ink droplet. Accordingly, by depositing the ink droplets on a recording paper sheet, an image or characters can be created.
Here, the two portions of the heating element 113 can be independently driven. The two portions are concurrently heated. If the periods of time in which the temperatures of the two portions reach the boiling temperature of ink (i.e., bubble generating time) are the same, the quantities of ink on the two portions boil at the same time. As a result, an ink droplet is ejected in a direction perpendicular to the nozzle sheet 117 (i.e., direction of the central axis of the nozzles 118).
In contrast, if the bubble generating times for the two portions are different, the quantities of ink on the two portions do not boil at the same time. As a result, an ink droplet is ejected in a direction offset from the central axis of the nozzles 118. That is, the ink droplet is ejected while being deflected.
As described above, according to the technology discussed in Japanese Unexamined Patent Application Publication No. 2004-1364, the ejection direction of an ink droplet can be deflected. This deflected ejection can prevent white streaking of a printed image, thereby obtaining the improved print quality.
However, the state of the surface (ejection surface) also has an impact on the print quality. That is, when the ejection of ink is repeated many times, the ink is deposited on the surface of the nozzle sheet 117 around the nozzles 118. The deposited ink has an adverse effect on the ejection direction of an ink droplet. As a result, the ink droplet is not deposited on the desired location of the print paper sheet, thereby degrading the print quality.
Additionally, if the ink deposited on the nozzle sheet 117 becomes solidified, the ink remains adhered to the nozzle sheet 117. If the adhered ink is removed from the nozzle sheet 117 and clogs the nozzle 118, the clogged nozzle causes an ejection defeat, and therefore, the print quality is degraded.
Accordingly, a technology has been proposed in which the nozzle sheet 117 has a hydrophobic area to prevent the deposition of ink (refer to, for example, Japanese Unexamined Patent Application Publication No. 8-39817). According to this technology, the nozzle sheet 117 includes a wiping mechanism to wipe the surface of the nozzle sheet 117, a hydrophobic area on the surface of the nozzle sheet 117 around the nozzles 118, and a hydrophilic area on the surface of the nozzle sheet 117 only downstream in the wiping direction.
According to the technology discussed in Japanese Unexamined Patent Application Publication No. 8-39817, the hydrophobic area provided on the surface of the nozzle sheet 117 can prevent the deposition of ink upstream in the wiping direction. Accordingly, clogging of the nozzles 118 due to the insertion of the adhered ink into the nozzles 118 by the wiping operation can be prevented. As a result, the ejection defect of the nozzles 118 can be prevented, thereby improving the print quality.
Also, a technology is proposed in which a plurality of U-shaped depressions is formed at positions slightly spaced away from the nozzle 118. That is, the surface of the nozzle sheet 117 provides a hydrophilic area, while a plurality of U-shaped depressions whose interiors are hydrophobic areas is formed at predetermined positions with respect to the nozzles 118 (refer to, for example, Japanese Unexamined Patent Application Publication No. 2001-1523).
According to the technology discussed in Japanese Unexamined Patent Application Publication No. 2001-1523, the hydrophilic area prevents the deposition of ink. The ink to be deposited on the nozzle sheet 117 is caught by the U-shaped depressions whose interiors are hydrophobic areas. Accordingly, the ink does not have a negative impact on the ejection direction of an ink droplet. As a result, the ejection defect of the nozzles 118 can be prevented, thereby improving the print quality.